Why can't we let each other go?

My ex I have known each other for years, we never ended things for bad reasons but we had just finished college and we were both going back home. We both have had relationships since than but we can't "let go" of each other. We will stop talking but we always come back to each other. He says he thinks about me all of the time and still wants to be there. However, we are both in relationships now. What do we do and why can't we just move on !?